Condensation Inside Bath Exhaust Fan Ducts?!? Help!

8 years ago

Hello. I've searched, but did not see any topic discussing how to fix this issue, or any preventive measures. Thanks in advance for your advice. I cannot figure how millions of homes are built, yet three homes in our neighborhood seem to have this problem.

At least three homes in this small 12 home neighborhood have the same issue. We had the same builder. Our floorplan is very different - the other two homes have very similar floorplans to each other I'll address our issue.

Home is located northern mid-atlantic, just east of the mountain foothills, maybe 1.5 hours west of the coast. Home is custom, built between March and September 2011. We've been moved in just over 3 years.
We opted to upgrade the insulation during construction, and got the new blown insulation (which we LOVE, btw). My husband says it's "blown in rockwood or fiberglass over spray-in closed cell". This is for all above-ground square footage.
Home has a main-level master.

The issue is with at least two bathroom exhaust fans. The fans vent through the roof. I don't think the exhaust fan ducting is insulated - it only goes up from the insulated space, through the colder attic barrier, then through the roof.
So ONLY during cold weather, we get condensation which forms on the INSIDE of the fan duct. This is causing ceiling drywall water damage where the exhaust fan cover meets the drywall, covering the fan blades.
During this last cold spell, temps got down to 5 to 15 degrees for several days. When we ran the bathroom fans, after we turned the fan off, we saw water. From one fan, the water would almost pour - drip very fast & quickly - directly from the fan vent, splashing onto the floor. That one wasn't so bad.
The MBR bath exhaust fan must have a different angle or some aspect of the flashing which allows the exhaust condensation to completely SOAK the surrounding ceiling drywall on that side of the fan. My husband took the fan cover off, and pushed up on the drywall - reminded me of rolled play-dough. There also were blisters of water, which when popped, the water splashed onto the floor tiles. It's bad.

One of our neighbors had his ducts "wrapped" trying to fix. Now we don't yet know WHAT the ducts were wrapped with, but he said this did not work. (our run isn't nearly as long as his - he has very high pitched roof w/no upstairs).

We have confirmed this is NOT the roof leaking. This is not rain coming back in from the flap thing (sorry, I'm not an HVAC lady :-). This is condensation.

So question to you Professionals please, please - How to most other new homes deal with this??? To me, wrapping the ducts with that fat pink insulation (the stuff with a silver skin on one side) would solve - but would it?
The fan seems strong, which I thought was good. Is that an issue??

The builder (punchlist after 3 years, does that give you any clue??) had the idea to rerun and incorporate a bend into the neighbor's duct, forcing a 'trap', and putting some type of tubular water collected there, then let the water run somehow to outside. (sorry, this sounds ludicrous to me).

What is Normal please? Thanks TONS for your ideas or known resolution, or sharing your Standards with us. This is crazy.......

Thanks & Regards - Les.

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Comments (3)

cold_weather_is_evil
8 years ago
last modified: 8 years ago
there are two separate issues that need to each be addressed entirely on their own. in all of this, remember that your fan duct is a chimney and reacts accordingly. you usually have warm air flow and waste even when not "using" it.

if you have condensation and it's all in cold weather, insulate the thing. Not with that trendy gimmicky thin plastic junk but with a whole lot of plain old fiberglass. a lot of it. reseal any seams first, perhaps with brush on goop and fiberglass mesh.

the water damage is a separate deal and I would not be any way qualified to suggest anything concrete without eyes-on. both issues do seem to be from poor installs. the s-trap is funny, though.
bpchiil
8 years ago
last modified: 8 years ago
If I am understanding you correctly, you are saying that the duct runs vertically from the fan to the roof? If that is the case, that is one of the biggest causes of your condensation issues.

Duct work should run horizontally for a short run of the duct work, so if condensation does collect (as it is in your case) it cannot find its way back to the interior of the house. Are your existing ducts flexible or metal ducts? If they are metal ducts, I would suggest a different run route as suggested, along with insulating the ducts with wrap. Wrap will work for you, provided you change the run
of the duct work.
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Ridgid is really the only way to go especially over long runs. We usually spec 4" sched. 20 PVC pipe as it comes in longer lengths than 4" metal does around here , though elbows can be tough to source sometimes. Unfortunately most contractors / HVAC guys will use flexible corrugated hose because it's cheap and fast. If it's a luxury project they'll splurge for the flex aluminum instead of the plastic kind. This type of pipe can cut a fan' effectiveness in half over long runs. Little thought is really given to these fans in most homes today. They are often undersized for a given bathroom and poorly located too. The crappy pipe detailing makes a bad situation worse. Panasonic's website has (or used to) a pretty good tutorial on how to properly size a fan, where to locate it and how to properly pipe it to the outside. I think much of the problem is cost and ignorance. Most people know you get a fan in your bath , but when pressed they won't really be able to tell you its there to expel warm moist air from your home, not smells. They assume that the $34.95 version will be fine for them and a quick & dirty install will get ya out the door for a hundred bucks. The reality is, many master baths, and some hard working kids baths need a much larger fan than 35 bucks can buy. We are often talking $100-$200 for an adequate model, more if you want features or quietness. Add the increased piping cost and labor for the proper venting needed on higher capacity fans and this can quickly become a $500 line item - which starts to look crazy for something as pedestrian looking as a bath fan. Timer switches are also a very good idea as most fans even properly sized ones need to run a while after you've left the bath to completely exhaust the moist air.
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I assume it isn't the strength of the fan, it is the vent line. The fan will exhaust 180cfm provided it is vented as intended. If you put in a reducer, you reduce the air flow. The flow will certainly be less than 180, but you are right that it will be over 55. As a lower limit, assume that 100% of the air that was flowing outside the 6" reducer will be stopped. Dropping from 8 to 6 cuts down the cross section of pipe to 56%. That would give a lower limit of 101 cfm. Of course, the air doesn't all just bounce back. It creates a line of higher pressure that increases the flow through the reducer above that lower limit. Not sure you are going to win an argument with the inspector though.
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Hmmm, that's interesting. We have a fan unit in the attic that vents 2 bathrooms that are side by side. From the unit, the vent duct goes straight up to the roof. You could certainly insulate the duct work, I am pretty sure we did that mostly for noise (we used rigid ducts and you hear the turbulence, not the fan). We have a cathedral type ceiling in both bathrooms, and we mounted the vent on the vertical wall above the shower head, routed to the motor, then as above (does that make sense to you?). This does not eliminate the heat loss into the duct, but as it is on a vertical wall, rather than the ceiling, maybe it is less like a "chimney", although I am sure we loose heat there too. This routing avoids running what sounds like a "heat conduit" between the roof and ceiling. But now I think I will go outside in the morning and look at our roof after over a foot of snow...
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I think you mean the vent duct is uninsulated? Another possible cause is not leaving the fan running for at least 30 minutes after showering/bathing. That's why many are installed with timer switches. You need to keep air running through the line for awhile so that the moisture that condenses in the ducting gets absorbed into the warmer and less-humid air that passes through afterward. It also helps to open a window, however slightly, in another part of the house so that you get adequate airflow out the vent. If your house is tight and there isn't an air opening to the outside, the vent won't be able to exhaust much air. (It's not like it can suck so hard as to create a vacuum in the house). Air will only exhaust to the extent the outflow is matched by an inflow.
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energy_rater_la
8 years ago
last modified: 8 years ago
one has to wonder if cfm of fan
is enough to push humid air
xxx feet UP.

this is why venting through the
side of house or soffit works
so much better.
this is allowed in my climate.
(aside from the fact of why would
you want a hole in the roof for
a bath fan??)

use a smooth vent, as spirals
will slow air flow & trap condensation
unless kept perfectly straight & tight.
pvc, sheet metal work well.
insulation would help, duct wrap
rather than batts. secure the duct
wrap with no gaps or voids, but
don't compress insulation.

best of luck.

Condensation Inside Bath Exhaust Fan Ducts?!? Help!

Comments (3)

cold_weather_is_evil

bpchiil

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energy_rater_la

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